Water purification typically produces a first effluent of relatively “clean water” and a second effluent of “waste water” which includes unwanted contaminants.
For example, the softening of hard water by the removal of calcium and magnesium is required for both industrial and household use. Known water softening processes proceed either by way of ion-exchange, membrane softening or precipitation. In the ion-exchange processes, the calcium (Ca++) and magnesium (Mg+) ions are exchanged for sodium (Na+) ions. Regeneration of the ion-exchange resin is achieved with a large excess of NaCl. This process creates a regeneration effluent being a relatively concentrated aqueous solution of sodium, calcium and magnesium chlorides. Consequently, by this method, considerable amounts of sodium, calcium and magnesium salts in solution must be disposed of.
Alternatively, it is possible to use weak acid resins which exchange hydrogen (H+) for calcium (Ca++) and magnesium (Mg++) ions, and to regenerate the spent resins with a mineral acid. While this method creates less waste water, it is more expensive and yields relatively acidic soft water which is corrosive.
Membrane softening concentrates the calcium, magnesium salts and salts of other divalent ions to produce waste waters which require costly disposal.
The precipitation process has traditionally been carried out by the “lime soda” process in which lime is added to hard water to convert water soluble calcium bicarbonate into water insoluble calcium carbonate. This results in waste water which is difficult to filter and requires use of cumbersome equipment.
The disposal of waste water has become an expensive problem for society. For example, approximately 1.61 billion gallons of waste water containing approximately 800,000 tons of mixed sodium, calcium, magnesium chlorides and sulfates is produced from water treatment operations and oil fields in the state of California alone. Disposal of this waste water costs millions of dollars each year.
It would be desirable for such waste waters to be processed so as to provide valuable products which could then be sold and used to offset the disposal costs.